According to the family of Eugene Stoner, one of the three men who engineered the first AR-15 military rifle, the gun designer didn't mean for the weapon to be used or owned by civilians.

"Our father, Eugene Stoner, designed the AR-15 and subsequent M-16 as a military weapon to give our soldiers an advantage over the AK-47," the Stoners told NBC News. "What has happened, good or bad, since his patents have expired is a result of our free market system."

The Stoners' comments were prompted by the renewed calls to control civilian access to weapons like the AR-15 in the wake of the shooting at Pulse, an Orlando gay nightclub, that left 49 people dead and 53 others injured.

Stoner's original AR-15 prototype was designed in his garage in the late 1950s with the idea of creating a gun that could pierce a metal helmet from 1,500 feet away, much to the liking of the U.S. military. The army quickly adopted the gun (which dubbed it the M16), which became standard issue for members of American armed forces and used to deadly effect in the Vietnam War. "After many conversations with him, we feel his intent was that he designed it as a military rifle," his family told NBC News. According to his family, Eugene Stoner never even owned an AR-15.

In 1997, after Stoner's death, a semi-automatic version of the gun became available to the public and quickly became a best-seller, leading the NRA to christen the gun "America's rifle."

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The main difference between the military-class and civilian-class AR-15s is a simple one: the former is fully-automatic, meaning that the trigger can be held once to fire continuously and the latter is semi-automatic, which has a triggering mechanism that must be pulled multiple times.

It's important to note that Omar Mateen, the Orlando gunman, used a Sig Sauer MCX, not an AR-15, in his attack. But like the AR-15, the MCX was designed for war: As Mother Jones reports, the MCX was originally created for the U.S. Army's special operations division that typically deals with unconventional warfare and counter-terrorism.

Gun advocates have gone to painstaking lengths to differentiate assault rifles, which allow the user to choose between fully-automatic and semi-automatic modes, and single-mode guns in an effort to argue why civilians can and should be able to purchase weapons like AR-15s and the MCX. As NBC News points out, though, semi-automatic AR-15s have been used in the 10 most recent mass shootings in the U.S., including the attacks in San Bernardino, Aurora, and Newton.

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"[Eugene] died long before any mass shootings occurred. But, we do think he would have been horrified and sickened as anyone, if not more by these events." Stoner's family said. "Currently, a more interesting question is, 'Who now is benefiting from the manufacturing and sales of AR-15s, and for what uses?'"